Consolidated nuclear steam generator

ABSTRACT

This invention relates to an improved system of providing power having a unique generating means of the nuclear reactor variety adapted with a plurality of steam generators in the form of replaceable modular units of the expendable type for the attainment of the optimum in effective and efficient vaporization of fluid during the process of generating power.

This is a division of Ser. No. 327,349 filed 1-29-73 now U.S. Pat. No.3,941,187 which is a continuation-in-part of application Ser. No.162,359 filed 7-14-71 and now abandoned.

In the art, there are many and varied power generating systems of thenuclear variety, some of which depend for their operability on thevaporization of fluids such as water. However, to insure predictabilityof operativeness, long term life and safety, these generating systemsare necessarily required to be assisted with a great many separate anddistinct auxiliary systems, each of which has an inherent requirementfor space due to their individual size and shape. In the last analysis,the effect of such an operative interweaving web of auxiliary systems isa somewhat large, cumbersome and comparatively inefficient plant inrelation to the size of the site required for its installation,maintenance and use.

For instance, in utility applications such as a central power station,the requirements could range in amounts up to 1200 megawatts electricalfor the more common unitary nuclear power generators which in the pasthave usually required as much as about 500,000 square feet of space formaintenance and use. And, if as the ecologists require, such plant wereforced to be placed in an underground concrete bed, the same plant wouldhave to be housed at depths as deep as 300 feet. Also, presently inother applications such as marine usage, the speed of a nuclear poweredvessel is inherently limited, amongst other factors, by the spaceavailable for the power generating system. As of the state of the art atthis point in time, the usual nuclear power generating system formaritime usage limits the power output to 325 megawatts thermal as itsupper limit primarily due to the overall spacial requirements of thereactor and auxiliary systems. Thus, the drive of such vessel is limitedto about 120,000 shp. In effect, as should be apparent, one of thedeterrents to the use of nuclear power in many specific applications isthe spacial requirements of the plant due not only to the reactor itselfbut also to the required auxiliary systems.

What is needed in the art is a plant having an arrangement in the formof a power generating system which does not inherently require the useof substantially all of the auxiliary systems of the art for itsoperation, maintenance and use in an effective and efficient manneraccompanied nonetheless by the optimum in the area of safety.

This invention answers the needs of the art relative to the threedimensional requirements of size and shape with special emphasis on anuclear power generating system which may be used to produce poweroutputs exceeding those of the art without the necessary use of and, ineffect, the substantial elimination of the following auxiliary systemsfrom the arrangement of the present invention, viz

(a) drives for partial-length control rods;

(b) core flooding system;

(c) injection systems of the high pressure variety;

(d) spray systems of a thiosulfate nature;

(e) feed and bleed systems of the soluble poison type;

(f) incore instrumentation systems;

(g) computer systems of enormous size;

(h) liquid waste disposal systems again of large size;

(i) seismic loop support systems;

As an attendant advantage of the elimination of the aforesaid, theperiodic testing and maintenance of such auxiliary systems with theirinherent requirements of space and cost are also eliminated.

It is therefore an object of this invention to provide an improvedsystem of the nuclear variety for the generation of power without anyinherent requirements in the system for the aforesaid auxiliary systems.

Another object is to provide an improved power generating unit of thenuclear variety having provision therein for replaceable steamgenerators of the expendable modular unit type.

A further object is to provide a steam generator of the modular varietyfor the attainment of the optimum in effective and efficientvaporization of fluid during the power generating process.

Other objects and many of the attendant advantages of this inventionwill become more apparent to one skilled in the art from the followingdetailed description taken with the accompanying drawings, wherein:

FIG. 1 is a schematic of the flow system of the present invention;

FIG. 2 is a cross-sectional view of the reactor of the present inventionhaving a unique arrangement of elements therein;

FIG. 3 is a section taken on line 3--3 of FIG. 2 showing a latticeconsisting of a plurality of steam generators around the periphery ofthe core of the reactor;

FIG. 4 is a view taken on line 4--4 of FIG. 3 showing an overallgrouping of a number of the individual modular steam generators of thereactor;

FIG. 5 is a view of an individual modular steam generator of the reactorof FIG. 2;

FIG. 6 is a view taken on line 6--6 of FIG. 5;

FIG. 7 is a view taken on line 7--7 of FIG. 5;

FIG. 8 is a side elevation in broken section of another heat exchangerthat embodies principles of the invention;

FIG. 9 is a side elevation of still a further heat exchanger thatembodies principles of the invention;

FIG. 10 is a side elevation in full section of a reactor pressure vesselfeedwater penetration; and

FIG. 11 is a side elevation of a portion of a reactor pressure vesselthat displays additional principles of the invention.

Similar numerals refer to similar parts throughout the several views.

One aspect of the subject invention, as shown in FIG. 1, in the broadsense involves a closed loop power generating system wherein a pluralityof steam generating units of the nuclear reactor variety are eachconnected in parallel to two coupling units which are part of andcommunicate in series within the closed loop of the system. Inaccordance with this arrangement, any one or a number of such reactorunits may be placed in flow sequence to provide a range of one to threetimes the power output of the system. For instance, if one unit provides400 megawatts, the result of placing the three units in paralleloperation in the closed loop system would be to supply as much as 1200megawatts. But, what is not apparent, is that this power output may beaccomplished without the auxiliary systems necessitated by the use ofthe common unitary nuclear reactor of the art having the same overalloutput.

More specifically, in the system of the present invention, each of theconsolidated nuclear steam generators 11 communicates with a couplingunit or mixing chamber 12 with appropriate valving therebetween for thetransmission of steam to a turbine 13 for the conventional generation ofpower. A condenser 14 is in series with the turbine 13 in proximity to apump 15 for transmission of condensate to a demineralizer 16 and thencein series through an air ejector 17, a low pressure heater 18, anddeaerator heater 19. A main pump 21 takes feedwater from the heater 19and feeds it to a high pressure heater 22 for transmission to theselected nuclear steam generator 11 through a coupling 23 andappropriate valving.

Other attendant advantages of the subject system include the fact thatthis type complex may be physically located on sites which could notpossibly accommodate the conventional unitary nuclear power generatingsystem of comparable output. The safety and siting advantages offered bythe subject system far exceed those of other systems utilizing lightwater reactors, and at the same time, the plant or system is easier tomaintain and operate, and produces negligible liquid wastes, besidesbuilding up a comparatively lower level of radioactive contamination.

Referring to FIG. 2, the consolidated nuclear steam generator 11 of thepresent invention is provided with a pressure vessel 26 having aconventional core 27 predominately made of a lattice of fuel elementswhich define primary coolant flow passageways within the core. A heatexchanger or steam generator 28 is in primary coolant flow communicationwith the vessel 26 and is provided with means 29 for circulating primarycoolant through the generator 28 and vessel 26 via such passageways. Thevessel is also provided with means for providing feed water 31 andextracting steam 32 each of such means being coupled to the aforesaidclosed loop system as shown in FIG. 1.

The essence of the system and the reactor arrangement is that a uniquesteam generator is in spaced relationship above the core in the pressurevessel 28. The flow path of the primary coolant is as indicated bypoints 35 to 39 in FIG. 2. And, as shown, each of the steam generatormodular arrangements is provided with a support plate 41 on the bottomthereof and a supporting wall 42 extending longitudinally along theinterior of the same which will be hereinafter described in detail.

Previous efforts to develop a pressurized water reactor of the integralvariety as shown in FIG. 2 have been complicated by a characteristicincompatibility between the respective shapes and dimensions of majorcomponents such as the core, core barrel and steam generator. Even aftersolving some of the problems of shape to provide a more compact reactor,it was found that the arrangements of the art increased manufacturingcosts and added problems associated with maintenance and in-serviceinspection. Therefore, a reactor arrangement such as that shown in FIG.2 was sought in order to provide the desired degree of compactness butwith the avoidance of the added cost and inspection problems.

Improved compactness has been realized with the arrangement shown inFIG. 2 because of the steam generator 28 and its inherent shape andsize. As a result, the height and diameter of the pressure vessel bothwere dramatically reduced with a corresponding reduction in primarycoolant inventory to as little as 50 per cent of that required by theconventional unitary reactors of the art and the subject arrangement isrelatively free of thermal stresses caused by operating transients.

Other attendant advantages are that steam generators are in spacedrelationship above the core. In practice, the space should be at leastabout 24 inches in total extent. In this manner, the production ofradioactive oxygen is avoided in the secondary system with its attendantproblems which include the use of expensive shielding throughout suchsystem. Also, as an added advantage, activated corrosion products arealso avoided in the secondary system. In the past, this usually occuredwhen the metallic tubing came in contact with a neutron flux from thereactor core. As a result of the aforesaid advantages, ferritic steelmay be utilized in the fabrication of the steam generator which iscomparatively cheaper and which possesses a thermal conductivity greaterthan that of stainless steel for the optimization of the efficiency ofthe heat exchange process. In the past, the use of such material was notpossible. This was primarily because as the distance between the steamgenerator and the core decreases, the usual tendency of the ferriticsteels to crack and fail is increased. Also, as in the past, whenauxiliary systems were used which contain boric acid in the primarycoolant, the corrosion rate of the ferritic steels prohibited its use.Now however, such material may be used and due to the greater thermalconductivity of such materials, the steam generators may be fabricatedin smaller and cheaper units without having any compromising effect onthe overall required heat transfer of such unit. Further, if as in thepast, the steam generator were adjacent to the core, the steam flow pathwould present substantial technical, operational and maintenanceproblems. However, now, with the arrangement shown in FIG. 2, there is arelatively low resistance to flow in the primary system and a naturalcirculation path is maintained even though the pump 29 fails to operate.In fact, the circulation of the subject reactor in such case would besomewhat more intensified than that of the natural overall circulationor flow ordinarily encountered in reactors of the art. Thus, unusuallygood performance in this respect is encountered even with inoperativecirculating pumps.

In the present invention, the heat exchangers or steam generating units28 for use in the reactor assembly are in the form of a plurality ofseparate and replaceable modular units each of which are expendable andthe total of which control the heat transfer process of the system and,as described, the flow distribution of the coolant through such unit. Asis apparent from the art, the output of a nuclear reactor is limited,amongst other factors, by the rate at which heat can be transferred tothe secondary fluid flowing within the heat exchanger.

One structural form for providing the optimum in effective and efficientheat transfer between a primary and secondary fluid within the aforesaidreactor is that in which a plurality of elongated heat exchange tubescontaining a secondary fluid are packaged within a prescribed volumetricarea in a definite array in the flow path of the heated primary fluid.The primary fluid in such a case flowing in a downwardly direction overa maximum surface of the exterior area of the tubes, transferring itsheat to the secondary fluid Within each of the tubes, and such secondaryfluid being easily vaporized producing steam or gas. The latter vaporwill then flow in a countercurrent or upwardly direction within suchtubes relative to the flow of the primary fluid. To provide the optimumto the system relative to heat transfer, the bundles of tubes arefabricated in groups 40 called modular units, which are positioned inadjacent relation around the periphery of the core as shown in FIG. 3 inthe overall form of a lattice 41.

The specific lattice shown in FIG. 3 consists of a plurality ofuniformly constructed, elongated and longitudinally contiguousassemblies of individual steam generating units.

However, the overall configuration of the lattice may be somewhat variedin arrangement without departing from the spirit of this invention aslong as each of the steam generators 28 which make up the lattice 41 isan individual modular unit of the separable and replaceable varietyhereinafter more fully described. Further, as shown in FIG. 4 to provideintegrity to the free-flowing nature of the primary fluid through thelattice or overall steam generator assembly, the individual groupings 42of the steam generators are positioned at varying elevations relative toone another providing a substantial degree of unobstructed flow for theprimary fluid into, through and out of the steam generator assembly asshown by flow lines 43. A system of manifolds 44, as shown, are providedto transfer the steam from the modular groupings through the wall of thevessel 26 to conduits 32 of the overall power generating system as shownin FIG. 1. It should be noted that the enlarged portions of each of theindividual tubings 47 are positioned in lateral array relative to eachother across the center of the modular groupings 42 even though each ofthe steam generating units of the grouping are positioned at varyingelevation. In effect, the entire assembly of steam generators form aplurality of longitudinal flow channels for the primary fluid. Althoughthe modules are positioned at different elevations, the central unswagedportion of the individual swaged tube lengths are positioned at the sameelevation for all modules. This arrangement maintains the effectivenessof the heat transfer surface by avoiding a tendency for bypassing of anyindividual tube by the flowing primary fluid.

As shown in Fig. 5, the individual modular unit or separate steamgenerating unit 28 are essentially a unitary package of elongated tubes47A accurately located and retained in position relative to each otherwith reasonable freedom therebetween to expand with temperature change.The bundle of longitudinally extending tubes are stably supportedbetween two tube sheets, 50, 51 each of which form the basis, inconjunction with other elements, for the inlet and outlet header of theunitary system and which maintain the bundle from chatter and vibrationwhen the unit is subject to a heated turbulent flowing primary fluid. Ingeneral, the unit is laterally stabilized at intervals along its lengthby a structural unit called a grid 52. The latter eliminates the risk ofbowing or displacement of the individual tubes as a result oftemperature change or turbulent flow. In some instances, the grid couldbe formed by a plurality of interfitting straps to provide a lateralstructural network of openings for the reception and maintenance of thelongitudinal tubes in position in a lattice type configuration withoutobstruction of flow of the primary fluid over the individual tubes. Asindicated, the latter fluid flows longitudinally along and among thetube assembly as a vehicle for heat or energy transfer to the individualtubes.

As shown in FIG. 5, the individual relatively thin vertical tubes extendthrough both the upper and lower horizontal tubesheets side by side atpredetermined distances from one another. The outer face of either ofthe tubesheets or laterally joining means are preferably of a flatplanar nature at the points of contact of such tubesheets with theterminal ends of the individual tubes. In effect, as described, the unitrepresents a compact package of nested tubes. The methods of fasteningor securing such tubes to tubesheets are numerous and includebeam-welding or any other common or conventional method of the art forsuch desired purpose.

A further advantage, as shown in FIG. 2, is the improved accessibilityof the steam generator section 28 in the reactor itself due to theability to remove the entire interior structure located above the coreas a unit. The latter unit being constructed of outer walls 42A whichsupport the steam generator and are braced longitudinally by stiffeningribs 43A which in turn are vertically supported by plates. After theinterior unit described is removed, access may be easily accomplished tothe individual groupings of the steam generators. In this manner, eachof the modular units may be either easily replaced, expended, or everrepaired in situ. Also, on a comparative basis, relative to the art ofrecord, less time is required to fully remove and replace each of thesteam generating modular units. Also, tube leaks in the individual tubesin a modular unit may be easily isolated with less loss of capacity on acomparative basis.

In a preferred embodiment, the modules would have a square cross-sectionand a pitch compatible with the core fuel assembly size and pitch. Inessence, the once through steam generator with its low secondaryvelocities and upward flow was found to be uniquely adapted to themodular form required. Preferably, the individual tubes of the modularunit have a wall thickness of about 0.067 inches and body portion withan outside diameter of about 0.50 inches terminating at either end in aswaged manner to an outside diameter of about 0.370 inches. The pitch ina practical unit is about a 0.611 inch square. Primary coolant flow inand out of each of the modular units if facilitated by the swaged tubesection. And, the characteristically small tube diameter of the modularunits reduces the leak rate resulting from a tube failure, andcontributes to compactness besides minimizing the requirement for thickwalls for the tubes themselves.

The individual headers 52,53 as shown in FIGS. 6 and 7 are either of aforged or cast nature. These individual pieces are welded to thetubesheet into the position shown in FIG. 5. Each of the headers isprovided with a peripheral border 54 which is engaged to the tubesheet.The interior of the main body section of each of the headers issupported against the external pressure load of the primary fluid by aplurality of uniformly spaced columns 55 which rest against thetubesheet when the border is welded into position. Since the internalpressure is considerably lower than the external pressure, it is notnecessary to weld each of these columns at the contact surface of thetubesheet.

Preferably, the steam generator modules are manifolded in groups of nineand ten. And, as previously shown, are located in at least threevertical positions relative to each other to allow unrestricted flow ofthe primary fluid. Also, each module is free to expand or contractvertically during operating transients by virtue of the flexibility inthe manifold piping itself. Further, the penetrations of the vessel forfeedwater leads and steam outlets are generally symetrical in eachquadrant of the vessel.

In operation, primary water flows longitudinally downward over the steamgenerator tubes at about the same velocity as that existing in the core.Also, feedwater or the secondary fluid is provided by the inlet feed tothe feedwater tube 56. The feedwater then flows downwardly to the lowermodule header and is dispersed in a uniform pattern from the lowerheader into each boiler tube and is evaporated and superheated as itflows vertically upward through tubes 47 to be collected in the upperheader through the opening 60 to the manifolds of the power generatingunit of the system. Experimental results have shown that the modularsteam generator will be free of critical or unusual stability problemswith or without swaging of the individual boiler tubes.

In the system heretofore described, tube leaks may be isolated withoutinterrupting plant operation. Under compression, the tube walls of thegenerator are less affected by defects and less subject to stresscorrosion cracking. The tubes can operate at lower stresses and shouldnot leak even if they collapse. Also, the individual steam generators donot require the special shop facilities that a conventional unitrequires. Thus, the modular unit is well adapted to mass productiontechniques not previously applicable to the manufacture of steamgenerators of the pressurized water variety.

The extreme compactness of the system heretofore described relative tooutput, result in significant savings for such applications as themanufacture, maintenance, and use of a central power station. Inaddition, there are fewer and less elaborate auxiliary and safetysystems required by such a system which tend to increase the expense ofan ordinary conventional system.

A further embodiment of the invention, shown in FIG. 8, includes ashrouded feedwater inlet tube 100 that penetrates, in a slidingrelationship, the tube sheet 101 and the steam outlet header 102 toestablish fluid communication with a feedwater inlet chamber 103 that isformed between an inlet tube sheet 104 and a feedwater header 105. Ithas been found that a shroud 106 inhibits steam formation within aninwardly disposed, concentrically positioned feed tube 107. This featureof the invention serves to control possible instabilities in steamgeneration and fluid flow within the heat exchanger module.

As shown in the drawing, the shroud 106 is confined in recesses that areformed in the surfaces of the tube sheets 101 and 104. The feed tube107, in contrast, passes through penetrations formed in both of the tubesheets. The feed tube 107, moreover, is firmly secured in the tubesheets through belling, welding, or the like.

An additional feature of the invention is provided by spacer grids 108.These grids, of a type that is described for example In U.S. Pat.Application Ser. No. 207,255 filed on Dec. 13, 1971, by Felix S. Jabsenare provided with resilient detents 109 that protrude inwardly towardthe tubing that forms the heat exchanger. The detents engage thesurfaces of heat exchanger tubes with sufficient force to inhibit tubevibration at high flow velocities. Thus, for instance, in conventionalheat exchangers the flow velocity of the fluid outside the heatexchanger tubing is limited to about five feet per second. At highervelocities tube vibrations commence that tend to promote failures and,in general, shorten the life of the heat exchanger. With the Jabsenspacer grids, however, the flow velocity can be increased by a factor ofat least three, to a velocity of not less than fifteen feet per secondwithout generating appreciable vibration or shortened module life.

The grids 108 can be secured to the tubes in the bundle throughinserting and rotating keys (not shown) that temporarily move thedetents 109 away from the tubes during assembly before positioning thegrid on the tubes. In this way, the deflected detents protect the tubesfrom scratching and marring. Clearly, heat exchanger assembly is madeless expensive through the application of this technique that isdescribed in the aforementioned Jabsen patent application in connectionwith fuel element assembly. After the grids are properly positioned onthe heat exchanger tubes, the keys are once more rotated and thenwithdrawn from the grid and heat exchanger assembly.

An additional embodiment of the invention is shown in FIG. 9. Typicallya steam manifold 111 is attached directly to the inside wall 112 of thereactor pressure vessel. A feed manifold 113, however, is spaceddirectly above the heat exchanger module group 114. Coiled feedwatermanifold tubing 110 between the header 113 and the individual modules inthe group 114 allow for differential expansion of the tubing withrespect to the module, internal steam manifolding 115 and the pressurevessel.

In passing, it should be noted that modular groups 114 close to thepressure vessel wall are supported largely through the tubes thatcomprise the internal steam manifolding, rather than on the supportplate 41 shown in FIG. 1.

As shown in FIG. 10, an inlet feed line 116 external to reactor pressurevessel 117 must penetrate the vessel in order to deliver fresh feedwaterto the heat exchanger modules. Because the incoming feedwater isrelatively cold, in contrast to the higher temperature thick-walledreactor pressure vessel, not only is there a risk of initiating boilingin the water that flows through the inlet, but there also is a risk ofestablishing inacceptably high stresses within the reactor vessel wallbecause of temperature related differences in the expansion of the metalin the vessel that is adjacent to and spaced from the cold feedwaterinlet.

In order to cope with this problem feedwater inlet penetration apparatus120, built in accordance with the invention has a fluid-tight clamp 121,for example, a "Grayloc" clamp, to join the inlet feed line 116 to anipple 122 that is formed in the outer surface of the pressure vessel117. Illustratively, the nipple 122 is formed by boring out a recess 123in the vessel surface and filling this recess with a "puddle weld" 124of inconel that is machined to provide the re-entrant shape shown inFIG. 10 of the drawing. A fitting 125 that matches a prepared surface126 on the machined puddle weld 124 is joined to this surface through afurther weld, or the like.

The fitting 125 also has a long tube 127 that protrudes through apenetration 130 that is formed in the reactor vessel 117. The tube 127is much longer than the encircling part of the fitting 125 that isjoined to the prepared surface 126 of the machined puddle

The tube 127 extends through the penetration 130 to the inner wall ofthe vessel 117. The tube 127, moreover is concentric with and spacedfrom the surface of the penetration. A sleeve 131 of insulatingmaterial, e.g. several laminations of metal foil insulation, is lodgedin the annulus that is formed between the penetration 130 and the tube127. The sleeve 131 extends from the inner wall of the vessel 117 to aslight distance beyond the plane of the vessel's outer wall in order tonest within a recess 132 that is formed in the fitting 125 in order toserve as a thermal barrier between the vessel and the feedwater.

The fitting 125 terminates in a flange 133 that is outside of the planeof the outer vessel wall. Another similar, albeit oppositely disposed,flange 134 terminates the inlet feed line 116 and is in alignment withthe flange 133. The flanges 133 and 134 are separated by means of aninterposed seal 135 that has an orifice 136 concentric with the line 116and the tube 127. This orifice, it has been found, provides improvedfeedwater flow stability.

A tubular thermal shield 137 is secured to the concave surface of theorifice. The shield 137, moreover, is concentric with and extendsthrough the tube 127 in order to protrude beyond the inner wall of thevessel 117. A welding head (not shown) penetrates the interior of thetube 127 from the outer surface of the vessel 117 and completes the weldat the piece 140 from the interior of the tube 122. In a similar mannera cutting tool also can be introduced to cut the weld at the piece 140and facilitate the removal of the penetration apparatus 120, to befollowed by the insertion of a new apparatus.

The feedwater inlet penetration apparatus 120 terminates within thereactor vessel in a transition piece 140 that is formed by upsetting theend of a feedwater line 141 and welding the upset end to the terminalsurface of the tube 127.

Thus, there is provided a pressure vessel feedwater penetrationapparatus that promotes flow stability by forestalling the onset ofboiling within the feedwater tubing through the thermal shield 137. Thethermal shield also incorporates the flow-stabilizing benefits of theorifice 136. This advantageous feature of the invention is furtherenhanced through the clamp 121 that joins the feed line 116 to thepenetration apparatus 120. The clamp can be removed with relative easeto aid inspection or to replace failed tubing as the need arises. Thus,inspection and preventitive maintainence become much less expensive andtroublesome with the concept of the modular heat exchanger than with theusual designs that are characterized by a few large welded ducts andpipes. A like apparatus can be used for steam penetrations. It alsoshould be noted that to improve corrosion resistance under primary waterconditions, the entire internal surface of the vessel 117 should be cladwith inconel, stainless steel or some other suitable material. Thesurface of the penetration 130, moreover, should in this case be linedwith a similar corrosion resistant cladding.

Turning now to FIG. 11, a pair of steam inlet penetration apparatuses142 are joined together in a return bend 143. The combined flow to thereturn bend 143 then flows through a tee 144 that is coupled to amanifold 145. Enhanced safety is one of the salient characteristics ofthis feature of the invention. For instance, in the event of a linefailure, the high pressure of the steam ordinarily would cause the freeend of the parted line to whip about and spray hot, pressurized steamabout the plant.

In the event one of the lines coupled to the return bend 143 parts,however, the unbroken line will tend to restrain the otherwiseunrestricted movement of the parted tube, thereby reducing theaforementioned hazards associated with an unsecured tube. Naturally, asimilar advantage obtains through the use of a return bend in connectionwith feedwater manifolds.

A further advantage that attends the modular heat exchanger designherein described is the opportunity for extensive use of relativelysmall diameter tubing. Thus, tubing within the realtor vessel 117 of adiameter that is relatively large, of more than one inch (iron pipesize, drips), for example, must satisfy very stringent safety codetests. Smaller diameter tubing of the sort that is used extensively inthe practical application of this invention need not satisfy suchrigorous test requirements. In a similar manner, piping external to thereactor and exposed to the atmosphere that has a diameter of four inchesIPS or less, need not satisfy such difficult and expensive teststandards. As a consequence, the extensive use of small diameter tubingsignificantly reduces inspection costs without impairing the safety ofthe system.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A steam generatormodular arrangement which comprises: a support plate; a plurality ofheat exchangers, each of which is supported at one end by said supportplate; a manifold; a plurality of flow lines, each of which is connectedat one end to said manifold and at the other end to one of said heatexchangers; said heat exchangers being positioned longitudinallystaggered relative to one another; said heat exchanger having a steamheader, a feedwater header spaced from said steam header, a pair of tubesheets interposed between said headers and each joined to one of saidheaders to form a steam chamber and a feedwater chamber respectively, afeedwater tube passing through said steam chamber to establish fluidcommunication with said feedwater chamber, a plurality of steamgeneration tubes longitudinally disposed between and secured to saidtube sheets to establish fluid communication between said feedwater andsteam chambers, each of said steam generation tubes including a largerdiameter central tubular portion terminated at each end by a smallerdiameter tubular portion, a plurality of transversely disposed spacergrids in engagement with said steam generation tubes, resilient detentsformed on said spacer grids to engage surfaces on said steam generationtubes and restrain the vibration thereof, wherein said steam chamber isin fluid communication with one of said flow lines; and a shroudconcentric with and disposed outside of said feedwater tube, said shroudbeing secured in said tube sheets.
 2. A steam generator arrangementwhich comprises: a support plate; a plurality of heat exchangers, eachof which is supported at one end by said support plate; a manifold; aplurality of flow lines, each of which is connected at one end to saidmanifold and at the other end to one of said heat exchangers; said heatexchangers being positioned longitudinally staggered relative to oneanother; said heat exchanger having a steam header, a feedwater headerspaced from said steam header, a pair of tube sheets interposed betweensaid headers and each joined to one of said headers to form a steamchamber and a feedwater chamber respectively, a feedwater tube passingthrough said steam chamber to establish fluid communication with saidfeedwater chamber, a plurality of steam generation tubes longitudinallydisposed between and secured to said tube sheets to establish fluidcommunication between said feedwater and steam chambers, each of saidsteam generation tubes including a larger diameter central tubularportion terminated at each end by a smaller diameter tubular portion,said larger diameter central tubular portions of said steam generationtubes in each of said longitudinally staggered heat exchangers beingpositioned in lateral alignment relative to each other, a plurality oftransversely disposed spacer grids in engagement with said steamgeneration tubes, resilient detents formed on said spacer grids toengage surfaces on said steam generation tubes and restrain thevibration thereof, wherein said steam chamber is in fluid communicationwith one of said flow lines; and a shroud concentric with and disposedoutside of said feedwater tube, said shroud being secured in said tubesheets.